As a commercial mushroom farm, we generate a large amount of “spent” blocks.  These are our sawdust-based fruiting blocks that have gone through the fruiting chamber and have produced two harvests, or “flushes” of mushrooms.  At  this point, the mushroom mycelium has extracted most of the available nutrients from the fruiting media, and broken down some of the cellulose and other polysaccharides found in the wood into simpler bioavailable compounds.  

Normally, as we start to accumulate a big pile of spent blocks there are lots of local farmers and gardeners eager to take it away by the truckload.  These blocks make a good “brown” component in a healthy compost, and can contribute to the development of a healthy living soil.  However, we’re always looking for ways to use this material ourselves as well, especially for further mushroom production.  

After reading some research on “re-inoculating” spent blocks with fresh mushroom culture in order to fruit them again, we decided to try this ourselves.  The process was as follows: we stripped the bags off of the spent substrate, mixed them with fresh wheat bran (our standard nutrient additive), added water to hydrate, bagged this new mix, and steam pasteurized it to remove unwanted competitor organisms.  We then inoculated this recycled mix with blue oyster mycelium (oysters are generally considered the species most able to adapt to diverse substrates) and incubated for ten days to allow the mycelium to grow through the substrate.

In our fruiting trials, we found a general reduction in yield of around 25% in the recycled blocks compared to a control of blue oyster fruited with fresh substrate.  In our specific case, this was too great of a difference to continue using this production method, mainly because we still had the fixed environmental costs of single-use plastic bags and propane for the creation of these new, less productive blocks.  However, there might be circumstances where this approach would be suitable, for example in situations where wood or other fibrous material is a limiting factor.

There’s a bit of an epilogue to this trial, which stretches into some work that is still ongoing.  After fruiting our recycled oyster blocks, we were left with some “double spent” blocks.  Noticing that this twice fruited substrate had a different texture and composition to our usual spent mix, we decided to use this material as a substrate for growing King Stropharia in an outdoor bed.  Normally, King Stropharia prefers a larger particle size for a more aerobic fruiting media,  but we thought that this mix might be appropriate as it was “soily’ and less woody than a straight sawdust bed.

I was a bit dubious about our chances of success, as usually when spent oyster substrate is made into a bed the existing mycelium will fuse and form a large oyster colony, excluding other fungi.  However, after a month this bed produced absolutely beautiful stropharia crops; the mushrooms were fat and almost porcini-shaped, with a distinct golden colour to the immature buttons.

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We are currently doing trials with other stropharia beds using spent substrate.  So far, the standard spent substrate is fairing less well, being more prone to fruiting oysters than stropharia.  However, work continues on this project in 2022.  The dream of a multi-step production system where one waste product becomes the starting point for another crop seems a little bit closer.  Check back for updates as the season progresses!